Electrical testing of prited circuit boards employing a multiplicity of non-contact stimulator electrodes

ABSTRACT

Apparatus for the electrical testing of electrical circuits including at least one array of non-contact stimulator electrodes having a multiplicity of individually controlled stimulator electrodes arranged to be linearly disposed adjacent a first side of an electrical circuit to be tested; a signal generator coupled to the at least one array arranged to supply an electrical stimulation signal to each of the stimulator electrodes; and at least two non-contact sensor electrodes, each having dimensions sufficiently large to overlay part of a conductor on the electrical circuit to be tested.

FIELD OF THE INVENTION

The present invention relates to equipment and methods for testing theelectrical integrity of electrical circuits, and more particularly toequipment and methods for the non-contact electrical testing of printedcircuit boards (“TCBs”), chip carriers and similar electrical circuitshaving conductors of various configurations.

BACKGROUND OF THE INVENTION

Electrical circuits, such as PCBs and chip carriers, are generallytested after manufacture to determine whether or not all of theconductors and other electrically conductive elements in the circuit arein their designated positions and to ensure that they are notunintentionally cut, shorted or otherwise have an undesired continuityor lack thereof. The conductors of electrical circuits are normallyinterconnected to define nets.

Conventional methods and apparatus for electrically testing electricalcircuits typically employ some kind of physical-contact with the nets.For example, in moving probe apparatus, a pair of probes may bephysically moved by an X-Y mechanism into and out of contact withterminals of various nets. Because nets are tested sequentially bymoving the probes from net to net, moving probe testing is a relativelyslow method for electrically testing complicated electrical circuits.

Another method for electrically testing electrical circuits employs aso-called “bed-of-nails” testing fixture. A bed-of-nails fixturetypically includes a large number of pins, which are positioned so thatwhen a circuit to be tested is pressed thereagainst, the pins come intoelectrical contact with pads at the terminal ends of each net toestablish electrical contact therewith. The conductivity of each net issubsequently measured. Although an electrical circuit can be tested muchfaster on an existing bed-of-nails fixture than by using a moving probe,bed-of-nails testing requires a dedicated fixture to be constructed foreach electrical circuit configuration. As a result, bed-of-nails testingis, overall, a time consuming and costly solution.

Electrical testing methods which rely on physical contact with anelectrical circuit to be tested, such as the moving probe andbed-of-nails methods described above, suffer from at least twoadditional fundamental disadvantages: First, as the size of pads at theterminal ends of conductors on electrical circuits decreases and theirdensity increases, it becomes increasingly difficult to obtain adequateelectrical contact therewith. Second, physical contact between conductorpads and the probes or pins may damage the pads.

To overcome these difficulties, a number of non-contact electricaltesting methods have been proposed. One non-contact printed circuitboard testing method is described in U.S. Pat. No. 5,218,294, issued toSoiferman. The patent describes stimulating a PCB under test with an ACsignal through power and ground lines or layers, or in a non-contactmanner by employing a near-field active antenna. The resultingstimulation generates an electromagnetic field which characterizes thePCB under test. The electromagnetic field proximate to the PCB undertest is measured in a non-contact manner and compared to theelectromagnetic field of a known faultless circuit board to determinewhether the PCB under test is defective.

U.S. Pat. No. 5,517,110, also issued to Soiferman, describes non-contactstimulation of a PCB by a pair of stimulators located adjacent to thePCB on one side thereof A resulting electromagnetic field is detectedusing a sensor array located between the stimulators on the same side ofthe PCB.

U.S. Pat. No. 5,424,633, issued to Soiferman describes a spiral loopantenna useful in the electrical testing of PCBs, as well as electricaltesting in which an electromagnetic field is applied to a first side ofa PCB under test by a non contact stimulator and an array of non-contactsensors on an opposite side of the PCB is operative to measure anelectromagnetic field that is characteristic of the PCB when stimulationis applied. This system is able to electrically test nets that haveterminal points on opposite sides of a PCB and relatively thin PCBs thatdo not have internal metal layers.

SUMMARY OF THE INVENTION

The present invention seeks to provide improved methods and apparatusfor non-contact electrical testing of electrical circuits such as PCBs.For the purpose of the description and claims which follow, anelectrical circuit being tested is referred to as a “board under test”or “BUT”.

One aspect of a preferred embodiment of the present invention providesfor the non-contact electrical testing of BUTs, such as PCBs, that havenets which begin and terminate on the same side thereof, and that haveother nets which begin and terminate on opposite sides thereof.

Another aspect of a preferred embodiment of the present inventionprovides for the non-contact electrical testing of BUTs, such as PCBs,that have internal metal layers and conductors that cross through orbetween the metal layers.

In accordance with a preferred embodiment of the invention, non-contactelectrical testing of BUTS, such as PCBs, that have nets which begin andterminate on the same side as well as nets which begin and terminate onopposite sides is performed generally simultaneously. One side of a BUTis stimulated with an AC electric field at a first frequency and theother side of the BUT is stimulated with an AC electric field at asecond frequency. Potentials induced by the different frequencystimulation in conductors on the BUT are measured and separatedaccording to frequency.

It is readily appreciated that by applying stimulation to both sides ofthe BUT that results in separable potentials that are identified withstimulation applied to one side or the other of BUT, the electriccontinuity in different types of conductors on a BUT can be testedsimultaneously.

In accordance with a still further aspect of the present invention, apattern of potentials on a BUT is analyzed and compared to a patterncharacteristic of an electrical circuit known to be not defective.

There is thus provided in accordance with a preferred embodiment of thepresent invention an apparatus for electrical testing of an electricalcircuit having first and second side surfaces and including a pluralityof conductors, the apparatus including at least one stimulationelectrode disposed adjacent at least one of the first and second sidesurfaces of the electrical circuit and being operative to apply theretoa stimulation electromagnetic field in a non-contact manner, at leastone sensing electrode disposed adjacent at least one of the first andsecond side surfaces of the electrical circuit and being operative tosense a resulting electromagnetic field produced by application of thestimulation. electromagnetic field at various locations thereon in anon-contact manner, wherein at least one of the at least one simulationelectrode and the at least one sensing electrode includes at least twoelectrodes at least one of which is disposed adjacent each of the firstand second side surfaces of the electrical circuit.

Further in accordance with a preferred embodiment of the presentinvention the at least one stimulation electrode includes at least firstand second simulation electrodes disposed adjacent respective ones ofthe first and second side surfaces of the electrical circuit.

Still further in accordance with a preferred embodiment of the presentinvention the at least one sensing electrode includes at least first andsecond sensing electrodes disposed adjacent respective ones of the firstand second side surfaces of the electrical circuit.

Further in accordance with a preferred embodiment of the presentinvention there is provided at least one stimulation signal generatorproviding at least one stimulation signal to the at least onestimulation electrode.

Additionally in accordance with a preferred embodiment of the presentinvention the at least one stimulation signal generator providesstimulation signals to a plurality of stimulation electrodes in a mannersuch that signals induced in the electrical circuit by individual onesof the stimulation electrodes may be distinguished from each other, andpreferably also includes at least one separating detector for receivingfrom the at least one sensing electrode signals induced in theelectrical circuit by individual ones of the stimulation electrodes anddistinguishes the signals from each other.

Additionally the apparatus for electrical testing of an electricalcircuit also includes a signal analyzer operative to analyze at leastone signal received from the at least one sensing electrode and acomparator receiving at least one signal derived from the resultingelectromagnetic field and operative to compare the at least one signalwith a reference.

Preferably the apparatus for electrical testing of an electrical circuitalso includes a defect report generator providing a defect reportrelating to the electrical circuit based on the output of thecomparator.

Additionally in accordance with a preferred embodiment of the presentinvention the at least one stimulation electrode includes first andsecond stimulation electrodes arranged to be disposed alongside a firstside of the electrical circuit and a third stimulation electrodearranged to be disposed alongside a second side of the electricalcircuit. Preferably the at least one sensing electrode includes a lineararray of sensing electrodes.

Still further in accordance with a preferred embodiment of the presentinvention the linear array is disposed intermediate the first and secondstimulation electrodes.

Additionally or alternatively the at least one stimulation electrodeincludes a linear array of stimulation electrodes.

Preferably the at least one sensing electrode includes first and secondsensing electrodes arranged to be disposed alongside a first side of theelectrical circuit and a third sensing electrode arranged to be disposedalongside a second side of the electrical circuit.

Further in accordance with a preferred embodiment of the presentinvention the at least one sensing electrode includes first and secondsensing electrodes arranged to be disposed alongside a first side of theelectrical circuit.

Still further in accordance with a preferred embodiment of the presentinvention the linear array is disposed intermediate the first and secondstimulation electrodes.

Moreover in accordance with a preferred embodiment of the presentinvention the at least one signal generator provides signals havingdifferent frequencies to different ones of the stimulation electrodes,and the at least one signal generator provides multiplexed signals todifferent ones of the stimulation electrodes.

Preferably the at least one stimulation electrode includes a pluralityof individually controllable sections.

There is also provided in accordance with a preferred embodiment of thepresent invention a method for electrical testing of an electricalcircuit having first and second side surfaces and including a pluralityof conductors, the method including the steps of applying anelectromagnetic field in a non-contact manner to at least one of firstand second side surfaces of the electrical circuit and sensing aresulting electromagnetic field in a non-contact manner at variouslocations along at least one of the first and second side surfaces ofthe electrical circuit, wherein at least one of the applying and sensingsteps employs non-contact electrodes disposed along both the first andsecond side surfaces of the electrical circuit.

Further in accordance with a preferred embodiment of the presentinvention the applying step includes employing at least first and secondsimulation electrodes disposed adjacent respective ones of the first andsecond side surfaces of the electrical circuit to apply at least oneelectromagnetic field thereto.

Preferably the sensing step includes employing at least first and secondsensing electrodes disposed adjacent respective ones of the first andsecond side surfaces of the electrical circuit to sense the resultingelectromagnetic field.

Still further in accordance with a preferred embodiment of the presentinvention at least one stimulation signal and is generated and providedto at least one stimulation electrode.

Additionally in accordance with a preferred embodiment of the presentinvention the generating step includes providing stimulation signals toa plurality of stimulation electrodes in a manner such that signalsinduced in the electrical circuit by individual ones of the stimulationelectrodes may be distinguished from each other. Additionally oralternatively the method also includes receiving signals induced in theelectrical circuit by individual stimulation electrodes anddistinguishing the signals from each other.

Preferably the method electrical testing of an electrical circuit alsoincludes analyzing at least one signal induced in the electricalcircuit,

Moreover according to a preferred embodiment, the present invention alsoincludes receiving at least one signal derived from the resultingelectromagnetic field and comparing the at least one signal with areference. Preferably the step also includes providing a defect reportrelating to the electrical circuit based on the comparing step.

Additionally according to a preferred embodiment of the presentinvention the applying step employs first and second stimulationelectrodes disposed alongside a first side of the electrical circuit anda third stimulation electrode disposed alongside a second side of theelectrical circuit.

Still further according to a preferred embodiment of the presentinvention the sensing step employs a linear array of sensing electrodes.The linear array may also be disposed intermediate first and secondstimulation electrodes.

Additionally according to a preferred embodiment of the presentinvention the applying step employs a linear array of stimulationelectrodes. Furthermore the linear array is disposed intermediate firstand second stimulation electrodes.

Preferably the sensing step employs first and second sensing electrodesdisposed alongside a first side of the electrical circuit and a thirdsensing electrode disposed alongside a second side of the electricalcircuit. Additionally or alternatively the sensing step employs firstand second sensing electrodes disposed alongside a first side of theelectrical circuit.

Preferably the method for electrical testing of an electrical circuitincludes a generating step in which signals having different frequenciesare provided to different ones of the stimulation electrodes.Additionally or alternatively the generating step includes providingmultiplexed signals to different ones of the stimulation electrodes.

Still further in accordance with a preferred embodiment of the presentinvention the applying step employs at least one stimulation electrodeincluding a plurality of individually controllable sections.

Additionally according to a preferred embodiment of the presentinvention also includes the step of grounding an intermediate metallayer in the electrical circuit.

Moreover in accordance with a preferred embodiment of the presentinvention the sensing step includes sensing potentials on one side ofthe electrical circuit and sensing potentials on the opposite side ofthe electrical circuit.

There is also provided in accordance with yet another preferredembodiment of the present invention a method for electrical testing of amulti-layered electrical circuit having first and second side surfacesand including a plurality of conductors, the method including the stepsof grounding an intermediate metal layer in the electrical circuit,inducing potentials into at least some of the conductors of theelectrical circuit, and sensing a resulting electromagnetic field in anon-contact manner at various locations along at least the first sidesurface thereof to obtain electromagnetic field data characteristic ofthe electrical circuit.

Further in accordance with a preferred embodiment of the presentinvention also includes sensing a resulting electromagnetic field atvarious locations along at least the second side surface thereof toobtain electromagnetic field data characteristic of the electricalcircuit.

Still further in accordance with a preferred embodiment of the presentinvention the electromagnetic field data is for the potential inconductors including the electrical circuit. Furthermore, the inducingstep may include inducing potentials on both a first side and a secondside of the electrical circuit.

Additionally in accordance with a preferred embodiment of the presentinvention the inducing step includes inducing potentials on the firstside of the electrical circuit which are differentiable from potentialsinduced on the second side of the circuit.

Moreover in accordance with a preferred embodiment of the presentinvention the inducing step includes inducing potentials which aredifferentiable by frequency.

Still further in accordance with a preferred embodiment, the presentinvention provides a method for electrical testing of a multi-layeredelectrical circuit wherein the inducing step includes inducingpotentials which are multiplexed.

Moreover in accordance with a preferred embodiment of the presentinvention the sensing step includes sensing electromagnetic field dataon one side of the electrical circuit. Preferably the sensing stepfurther includes distinguishing the electromagnetic field resulting frompotentials induced on the first side of the electrical circuit from theelectromagnetic field resulting from potentials induced on the secondside of the electrical circuit.

Further in accordance with a preferred embodiment of the presentinvention the inducing step includes inducing potentials on a first sideof the electrical circuit.

Additionally or alternatively the inducing step employs a plurality ofstimulators, and each stimulator induces potentials which aredifferentiable by frequency. Preferably the inducing step employs aplurality of stimulators, and each stimulator induces potentials whichare multiplexed.

Additionally in accordance with a preferred embodiment of the presentinvention the sensing step employs at least a first sensor and a secondsensor arranged along a first side of the electrical circuit. Preferablythe sensing step additionally employs a third sensor located along asecond side of the electrical circuit.

Moreover in accordance with a preferred embodiment of the presentinvention also includes correlating electromagnetic field data sensed bythe sensors to a stimulator. Additionally or alternatively a preferredembodiment of the present invention also includes determining electricalcontinuity of at least some of the conductors by comparing theelectromagnetic field data to reference electromagnetic field datacharacteristic of a desired electrical circuit.

Still further in accordance with a preferred embodiment of the presentinvention the inducing step is carried out in a non-contact manner.

There is further provided in accordance with a preferred embodiment ofthe present invention a method for electrical testing of a multi-layeredelectrical circuit having first and second side surfaces and including aplurality of conductors, the method including the steps of stimulatingthe electric circuit to induce in proximity thereto an electromagneticfield, acquiring electromagnetic field data in a non-contact manner atvarious locations along the first side surface, acquiringelectromagnetic field data in a non-contact manner at various locationsalong the second side surface, and determining electrical continuitycharacteristics of the conductors by analysis of electromagnetic fielddata for the first side surface and by analysis of electromagnetic fielddata for the second side surface.

Preferably in the method for electrical testing of a preferredembodiment of the present invention, the analysis steps employsreference data which is characteristic of an electrical circuit havingknown structure.

Still further in accordance with a preferred embodiment of the presentinvention the electrical circuit is a multi-layered circuit whichincludes at least one intermediate layer which is substantiallycompletely metalized, and the method includes grounding the at least onesubstantially completely metalized layer.

There is further provided in accordance with a preferred embodiment ofthe present invention a method for electrical testing of an electricalcircuit having a plurality of electrically conductive elements, themethod including the steps of applying a first electromagnetic field tothe electrical circuit with at least one stimulator located near but notcontacting the article on a first side thereof, applying a secondelectromagnetic field to the article at generally the same time as thefirst electromagnetic field with at least one stimulator located nearbut not contacting the article on a second side thereof, and separatelydetecting first and second potentials induced on the electricallyconductive elements of the article by the first and secondelectromagnetic fields, respectively.

Further in accordance with a preferred embodiment of the presentinvention the first and second steps of applying an electromagneticfield include the steps of generating electromagnetic signals of firstand second frequencies, respectively.

Still further in accordance with a preferred embodiment of the presentinvention the step or separately detecting includes the step of sensingthe potentials with at least one sensor located near the first side ofthe article. Preferably the method further includes the step of scanningby at least one sensor.

Additionally according to a preferred embodiment of the presentinvention the step of scanning includes the step of scanning in a firstscanning direction and followed by the step of scanning in a secondscanning direction which is substantially perpendicular to the firstscanning direction. Additionally or alternatively the step of scanningincludes the step of scanning the article in a first position followedby the step of scanning the article in a second position which isupside-down from the first position.

Preferably the method further including step of grounding internal metallayers of the article.

There is also provided in accordance with a preferred embodiment of thepresent invention an apparatus for electrically testing an articlehaving an electric circuit therein formed of a plurality of conductors,in which the apparatus includes (i) a first electromagnetic fieldgenerator applying a first electromagnetic field to the article, whereinthe first field generator includes at least one stimulator located nearbut not in contact with a first side of the article, and (ii) a secondelectromagnetic field generator applying a second electromagnetic fieldto the article, wherein the second field generator includes at least onestimulator located near but not in contact with a second side of thearticle, wherein the second side is opposite the first side, and (iii) asensor operative to separately detect first and second potentialsinduced on the conductors by the first and second electromagneticfields, respectively.

Further in accordance with a preferred embodiment of the presentinvention the sensor includes an array of sensors adjacent to the atleast one stimulator of the first field generator. Preferably the firstfield generator generates an electromagnetic field at a first frequency,and the second field generator generates an electromagnetic field at asecond frequency. Additionally or alternatively in the apparatus forelectrically testing an article, the first field generator includes afirst stimulator and a second stimulator operative to generate theelectromagnetic field.

Additionally according to a preferred embodiment of the presentinvention the first stimulator and second stimulator each generate afield which are 180 degrees out of phase with respect to each other.Preferably the stimulators are made of a plurality of strip-shapedelements.

Still further in accordance with a preferred embodiment of the presentinvention the strip-shaped elements extend obliquely relative to thearray of sensors.

Additionally or alternatively at least one of the stimulators is made ofa plurality of patch-shaped stimulators.

There is also provided in accordance with a preferred embodiment of thepresent invention a method for electrically testing an article having aplurality of conductors therein, which preferably includes the steps ofsubjecting a first side of the article to an electromagnetic field withat least one stimulator in close but not in contact arrangement with afirst side of the article, scanning the side of the article in at leasttwo partially orthogonal directions with a not in contact sensor,sensing potentials induced on the conductors by the electromagneticfield, and analyzing the potentials to determine the existence ofdefects in the elements.

Still further in accordance with a preferred embodiment of the presentinvention the method also includes the additional steps of subjecting asecond side of the article to a second electromagnetic field with asecond stimulator in close but not in contact arrangement with thesecond side, scanning the side of the article in at least two at leastpartially orthogonal directions with a not in contact sensor and sensingthe induction of potentials induced on the elements by the secondelectromagnetic field, and analyzing the potentials induced by thesecond electromagnetic field to determine the existence of defects inthe elements.

Additionally according to a preferred embodiment of the presentinvention the article is subjected to the first and secondelectromagnetic fields at generally the same time. Preferably theelectromagnetic fields are propagated at different frequencies.Additionally or alternatively the article includes a metal layer, andthe metal layer is grounded.

Moreover according to a preferred embodiment of the present inventionthe article is subjected to the first and second electromagnetic fieldsone after the other.

There is also provided in accordance with a preferred embodiment of thepresent invention a method for the electrical testing of an articlehaving a plurality of electrically conductive elements and internalconductive layers, the method including the steps of subjecting thearticle to an electromagnetic field with at least one stimulator inclose but not in contact arrangement with at least one side of thearticle, grounding the internal conductive layers of the article,scanning the at least one side of the article with a not in contactsensor and sensing the induction of potentials induced on the elementsby the electromagnetic field, and analyzing the potentials to determinethe existence of defects in the elements.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome apparent from the ensuing detailed description of the preferredembodiments, given by way of example only, when taken in conjunctionwith the drawings, in which:

FIG. 1 is a simplified pictorial illustration of non-contact electricaltesting apparatus, constructed and operative in accordance with a firstpreferred embodiment of the present invention;

FIG. 2 is a schematic cross-sectional illustration of a simplified BUTtogether with stimulators and sensors in accordance with a preferredembodiment of the present invention, taken along line II-II in FIG. 1;

FIG. 3 is a schematic circuit diagram of an exemplary circuit useful aspart of a separating detector forming part of the hardware of FIG. 1;

FIGS. 4A and 4B are simplified illustrations of unbroken and brokenconductors extending entirely along a first surface of a BUT in spatialregistration with a diagrams of potentials sensed thereon;

FIGS. 5A and 5B are simplified illustrations of unbroken and brokenconductors extending entirely along a second surface of a BUT in spatialregistration with diagrams of potentials sensed thereon;

FIGS. 6A-6D are simplified illustrations of unbroken and brokenconductors having a portion extending along a first surface of a BUT, aportion extending through intermediate parts thereof, and a portionextending along a second surface thereof, in spatial registration withdiagrams of potentials sensed thereon;

FIGS. 7A-7D are simplified illustrations of the unbroken and brokenconductors shown in FIGS. 6A-6D, but in upside-down testing orientation,in spatial registration with diagrams of potentials sensed thereon;

FIGS. 8A-8C are simplified illustrations of unbroken and brokenconductors having a portion extending along a first surface of a BUT, aportion extending through intermediate parts thereof, and anotherportion extending along the first surface thereof, in spatialregistration with diagrams of potentials sensed thereon;

FIGS. 9A-9C are simplified illustrations of the unbroken and brokenconductors shown in FIGS. 8A-8C, but in upside-down testing orientation,in spatial registration with diagrams of potentials sensed thereon;

FIGS. 10A-10B are simplified illustrations of two non-shorted and twoshorted conductors in spatial registration with diagrams of potentialssensed thereon;

FIGS. 11A-11B are simplified illustrations of the two non-shorted andshorted conductors shown in FIGS. 10A-10B but in upside-down testingorientation, in spatial registration with diagrams of potentials sensedthereon;

FIGS. 12 and 13 schematic illustrations of two alternative stimulatorconfigurations;

FIG. 14 is a simplified pictorial illustration of non-contact electricaltesting apparatus, constructed and operative in accordance with a secondpreferred embodiment of the present invention; and

FIG. 15 is a schematic circuit diagram of an exemplary circuit useful aspart of a separating detector forming part of the hardware of FIG. 14.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Reference is now made to FIG. 1 which is a schematic illustration ofnon-contact electrical testing apparatus, constructed and operative inaccordance with a preferred embodiment of the present invention. Testingapparatus 10 is operative to perform non-contact electrical testing ofelectrical circuits, such as are found on a BUT 12, having amultiplicity of electrical conductors 13. Although the present inventionis generally described in the context of non-contact electrical testingof printed circuit boards, it is readily appreciated that the methodsand apparatus disclosed herein are generally applicable to thenon-contact electrical testing of other electrical circuits including,for example, chip carriers, ball grid array substrates, multi-chipmodules, hybrid circuit substrates and printed circuit boards loadedwith electronic components. Reference herein to BUTs shall be deemed toadditionally refer to other suitable similar forms of electricalcircuits.

In a preferred embodiment of the invention, testing apparatus 10includes two first side stimulators 14 and 16 disposed to be adjacent toa first side of a BUT 12 and a first signal generator 18 supplying firstAC electrical stimulation signals to stimulator electrodes 14 and 16,hereinafter referred to as stimulators. A second side stimulatorelectrode 20, hereinafter referred to as stimulator 20, is disposed tobe adjacent to an opposite side of the BUT 12 and a second signalgenerator 22 provides second AC electrical stimulation signals thereto.An array of sensor electrodes 24, preferably comprising a plurality ofindividual sensors 25 arranged along a line, is preferably disposedbetween stimulators 14 and 16. Preferably, first side stimulators 14 and16 and sensors 25 are configured in as described in U.S. Pat. No.5,517,110, incorporated herein by reference.

A separating detector 26 receives the outputs of each sensor 25 andsupplies them to a signal analyzer 28 which outputs to a comparator andreport generator 30. As used herein, the terms “first side stimulators”14 and 16 and “second side stimulator” 20 relate to their respectivelocations as shown in FIG. 1. It is readily appreciated by those skilledin the art that the important factor relating to the position of thestimulators is that the two sets of stimulators are on geometricallyopposing sides of BUT 12. It is not of consequence which stimulators areabove or below BUT 12.

Preferably, the AC signals provided by first generator 18 and secondgenerator 22 respectively are different. For example, the AC signalsprovided by first generator 18 to first side stimulators 14 and 16 areat a first frequency F1 while the AC signals provided by secondgenerator 22 for second side stimulator 20 are at a second frequency F2.Preferably, first signal generator 18 provides signals to respectivefirst side stimulators 14 and 16 which are equal in amplitude and 180°out of phase with respect to each other.

It is readily appreciated that instead of distinguishing the AC signalsby frequency, the signals provided by first generator 18 and secondgenerator 22 may be at the same frequency but distinguished from eachother by suitable known signal separation techniques.

When energized by the AC electrical stimulation signals, first andsecond first side stimulators 14 and 16 and second side stimulator 20generate electromagnetic fields which stimulate BUT 12 and inducemeasurable potentials on conductors 13 of BUT 12. Each sensor 25 inarray 24 senses the potential induced in the conductors. Preferably, thepotentials are sensed by sensors 25 by capacitive coupling.Alternatively any other suitable sensing technique may be employed.

Preferably, the AC electrical simulation signals have a frequency in therange of 10 KHz to 20 MHz, and more preferably about 1 MHz. First sidestimulators 14 and 16 are preferably each as large as BUT 12 andseparated therefrom by an air gap typically 0.2 mm-2 mm, preferably asthin as possible due to mechanical limitations, or other suitableinsulating layer.

In a preferred embodiment of the present invention, BUT 12 and secondside stimulator 20, which is preferably sufficiently large to underlieall of BUT 12, are moved linearly past sensor array 24 and stimulators14 and 16. It is readily appreciated that alternatively BUT 12 andstimulator 20 may be held stationary while sensor array 24 andstimulators 14 and 16 are moved. Other combinations may also be suitableto scan BUT 12 with sensor array 24.

As is readily appreciated, the potentials induced in conductors 13 aredistinguishable from each other to the extent that the stimuli whichinduce these potentials are distinguishable from each other, for exampleby frequency or by multiplexing.

By employing information indicating potentials at various locations onBUT 12 sensed by sensors 25, signal analyzer 28 generates a preciserepresentation characteristic of potentials in conductors 13 on a BUT12, which indicates, inter alia, conductor continuity and which includesinformation regarding shorts and breaks in conductors constitutingdefects.

The representation provided by signal analyzer 28 to comparator andreport generator 30 enables provision of a defect report 34 indicatingdefective electrical continuity in conductors 13 of BUT 12 (FIG. 1),such as missing continuity where continuity is expected (opens and cutsin conductors) and excess continuity where not expected (shorts betweenconductors). The defect report preferably is generated by comparing therepresentation supplied by signal analyzer 28 with a reference 32representing a non-defective printed circuit board having the samedesign.

Reference is now made to FIG. 2, which is a schematic cross-sectionalillustration of a typical arrangement of conductors 13 of a simplifiedBUT 12 together with stimulators 14. 16 and 20 and sensors 25 (FIG. 1).In the illustrated example BUT 12 has a first surface 40 and a secondsurface 42, opposite to first surface 40, and comprises severalelectrical conductors 13, including:

(I) a conductor 50 located entirely along first surface 40;

(II) a conductor 52 located entirely along second surface 42;

(III) a first metal plane 54 located intermediate first and secondsurfaces 40 and 42, which is preferably grounded during testing;

(IV) a second metal plane 56 located intermediate first metal plane 54and second surface 42, which is preferably grounded during testing;

(V) a conductor 58, including a first portion 60 extending along secondsurface 42 and being connected through a plated via hole 62 to a secondportion 64 located intermediate first metal plane 54 and second metalplane 56, which is in turn connected through a plated via hole 66 to athird portion 68, which extends along first surface 40;

(VI) a conductor 70, including a first portion 72 extending along firstsurface 40 and being connected through a plated via hole 74 to a secondportion 76 located intermediate first metal plane 54 and second metalplane 56, which is in turn connected through a plated via hole 78 to athird portion 80, which extends along first surface 40.

It is appreciated that the there may be only a single metal plane or amultiplicity of metal planes, for example grounding planes, power planesor shielding, in a BUT. Conventionally, metal layers are not grounded,so that when a BUT is stimulated, a complex pattern of superimposedpotentials of the ungrounded metal layer and conductors is produced.However, if a metal layer separating the first and second surfaces ofthe BUT is grounded, a potential is not induced in the metal layer.Moreover, the metal layer normally isolates sensors from measuringpotentials in conductors in a non-contact manner on those portions ofthe conductors which are situated across the grounded metal layer fromthe sensor.

As described in greater detail hereinbelow with reference to FIGS.4A-11B, first side stimulators 14 and 16 stimulate conductors havingportions that extend above a first metal plane 54 and along firstsurface 40 adjacent thereto, such as conductors 50, 58 and 70 in FIG. 2.Similarly second side stimulator 20 stimulates conductors havingportions that extend below metal plane 56 and along second surface 42,such as conductors 52 and 58 in FIG. 2. It is a particular feature of apreferred embodiment of the present invention that by stimulating onboth sides of the BUT, all conductors having at least one portion thatis not sandwiched between two internal metal layers are stimulated andpotentials thereon are sensed.

It is appreciated that sensors 25 are able to sense potentials onconductor portions extending along first surface 40 or therebelow downto grounded first metal plane 54. Sensors 25 are not able to sensepotential on other conductor portions. In general, the BUT is preferablygrounded when performing electrical testing in various layers comprisinga multi-layered BUT. Thus, normally following testing of a BUT in theorientation illustrated in FIG. 2, the BUT is turned upside down andtested again, with sensors 25 adjacent second surface 42. When BUT 12 isin this orientation, sensors 25 are able to sense potentials onconductor portions extending along second surface 42 or therebelow downto second metal plane 56. It is appreciated that potentials on conductorportions lying intermediate second metal plane 56 and first metal plane54 normally cannot be sensed in a non contact manner when the secondmetal plane 56 and first metal plane 54 are grounded.

It is also appreciated potentials on conductors which extend entirely ina direction perpendicular to the scanning direction indicated in FIG. 2may not be adequately sensed. For this reason, normally followingtesting of a BUT in the orientation illustrated in FIG. 2, the BUT isrotated by 90 degrees and tested again, such that all of the conductorsare rotated by 90 degrees with respect to the scanning direction. It isthus appreciated that full testing of a BUT 12 preferably involves fourpasses through the apparatus of FIG. 1.

Reference is now made to FIG. 3, which is a schematic circuit diagram ofa preferred embodiment of a separating circuit 127 in separatingdetector 26 (FIG. 1). An output from sensor 25 is supplied to anamplifier 128, which outputs to first and second mixers 130 and 132which receive respective frequency inputs F1 and F2 from signalgenerators 18 and 22 respectively (FIG. 1). The outputs of mixers 130and 132 are supplied to respective low-pass filters (LPF) 134 and 136,respectively. The outputs of the mixers 130 and 132 are DC voltagesproportional to the amplitudes of the respective F1 or F2 frequencycomponents of the output from sensor 25 and a an undesirable AC out-bandsignal obtained as result from mixing of the components of thefrequencies F1 and F2. The LPFs 134 and 136 remove the undesirable ACsignal portion and provide the resultant DC voltages, one relating to asignal of frequency F1 and the other relating to a signal of frequencyF2, to signal analyzer 28, preferably via an AID converter (not shown).Preferably, a separating circuit 127 may be provided for each sensor 25in array 24, however, it is appreciated that signals from sensors 25 maybe multiplexed to a lesser number of separating circuits 127.

The first side stimulators 14 and 16 preferably generate signals of thesame frequency F1, but which are 180 degrees out of phase with eachother. The signal component at frequency F I sensed by sensor 25 at anyparticular sampling location is the sum of the potentials induced in theconductors 13 by stimulators 14 and 16. The amplitude and phase of thissignal component depends on the location of the conductors relative tothe locations of the first side stimulators 14 and 16. Alternatively,the first side stimulators 14 and 16 can be energized in-phase with eachother.

Reference is now made to FIG. 4A which shows electrical potentials,sensed by a sensor 25 (FIG. 2) lying above first surface 40 of a BUT 12,induced in a typical conductor, such as conductor 50 shown in FIG.2,which extends along first surface 40, by an electromagnetic fieldgenerated by first side stimulators 14 and 16 and second side stimulator20 in the arrangement of FIG. 2. In FIG. 4A, at least one grounded metallayer 90, such as a ground plane or a power plane, extends between firstand second surfaces 40 and 42 of BUT 12.

FIG. 4A includes a representation of conductor 50 arranged in spatialregistration with a first diagram 100 of the potential thereon, inducedby stimulators 14 and 16 and sensed by sensor 25, as a function of theposition along conductor 50 of the midpoint between stimulators 14 and16 along the scanning direction shown in FIG. 2 and a second diagram 102of the potential on conductor 50 induced by stimulator 20 as a functionof the position along conductor 50 of a sensor 25 along the scanningdirection shown in FIG. 2.

It is seen in diagram 100 that as the conductor 50 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on the conductor 50 induced bystimulators 14 and 16 goes quickly from a zero value up to a positivevalue +J and then decreases to a negative value −J, and thereafterreturns quickly to zero. It is appreciated that in this and otherdiagrams hereinbelow, if the phase relationship of the stimulators isreversed then the order in which positive and negative values areobtained is also reversed.

It is also seen in diagram 102 that as the conductor 50 is scanned inthe scanning direction by a sensor 25 lying above first surface 40, thepotential on the conductor 50 induced by stimulator 20 remains zeroinasmuch as conductor 50 is not stimulated by stimulator 20 since itextends only along first surface 40, which is isolated from stimulator20 adjacent to second surface 42 by at least one grounded metal layer90.

It is appreciated that conductor 50 shown in FIG. 4A is continuous andhas no breaks therealong.

Reference is now made to FIG. 4B, which is identical to FIG. 4A butrelates to a conductor 150, identical to conductor 50, except in that ithas a break at a location “i” therealong. FIG. 4B shows electricalpotentials induced in conductor 150, extending along first surface 40 ofa BUT 12 in the environment of FIG. 2, by an electromagnetic fieldgenerated by first side stimulators 14 and 16 and second side stimulator20 in the arrangement of FIG. 2. In FIG. 4B, at least one grounded metallayer 190, such as a ground plane or a power plane, extends betweenfirst and second surfaces 40 and 42 of BUT 12.

FIG. 4B includes a representation of conductor 150 arranged in spatialregistration with a first diagram 104 of the potential thereon inducedby stimulators 14 and 16 and sensed by a sensor 25 as a function of theposition along conductor 150 of the midpoint between stimulators 14 and16 along the scanning direction shown in FIG. 2 and a second diagram 106of the potential on conductor 150 induced by stimulator 20 as a functionof the position along conductor 150 of sensor 25 along the scanningdirection shown in FIG. 2.

It is seen that as the conductor 150 is scanned in the scanningdirection by stimulators 14 and 16 operating in a 180 degree out ofphase mode, the potential on the conductor 150 induced by stimulators 14and 16 goes quickly from zero up to a positive value +J and thendecreases to a negative value −J, and thereafter returns quickly to zeroat location “i”. From location “i”, in the scanning direction, thepotential on the conductor 150 induced by stimulators 14 and 16 againgoes quickly from zero up to a positive value +J and then decreases to anegative value −J and thereafter returns quickly to zero at the end ofconductor 150. It is appreciated that there is a clear and measurabledifference in the potential pattern produced in broken conductor 150 ascompared with that in continuous conductor 50.

It is also seen from diagram 106 that as the conductor 150 is scanned inthe scanning direction by a sensor 25, the potential on the conductor150 induced by stimulator 20 remains zero inasmuch as conductor 150 isnot stimulated by stimulator 20 since it extends only along firstsurface 40 which is isolated from second surface 42 by at least onegrounded metal layer 190. Thus inasmuch as conductor 150 does notinclude any portion extending below grounded metal plane, the stimulator20 does not have any effect in detecting a break in conductor 150.

Reference is now made to FIG. 5A which shows potentials, sensed by asensor 25 (FIG. 2) lying above first surface 40, induced in a typicalconductor, such as conductor 52 shown in FIG. 2, which extends alongsecond surface 42 of BUT 12, by an electromagnetic field generated byfirst side stimulators 14 and 16 and second side stimulator 20 in thearrangement of FIG. 2.

In the embodiment of FIG. 5A, at least one grounded metal layer 200,such as a ground plane or a power plane, extends between first andsecond surfaces 40 and 42 of BUT 12.

FIG. 5A includes a representation of conductor 52 arranged in spatialregistration with a first diagram 210 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along conductor52 of the midpoint between stimulators 14 and 16 along the scanningdirection shown in FIG. 2 and a second diagram 212 of the potential onconductor 52 induced by stimulator 20 as a function of the positionalong conductor 52 of sensor 25 along the scanning direction shown inFIG. 2.

It is seen that as the conductor 52 is scanned in the scanning directionby stimulators 14 and 16 operating in a 180 degree out of phase mode,the potential on the conductor 52 induced by stimulators 14 and 16 iszero, inasmuch as conductor 52 is not stimulated by stimulators 14 and16 since it extends only along second surface 42, which is isolated fromstimulators 14 and 16 by at least one grounded metal layer 200.

It is also seen that as the conductor 52 is scanned in the scanningdirection by sensor 25 lying above first surface 40, the potential onthe conductor 52 as sensed by sensor 25 remains zero inasmuch asconductor 52 does not have any portion that extends above grounded metallayer 200.

It is appreciated that in order for the system of FIG. 2 to test aconductor such as conductor 52 for continuity, BUT 12 must be turnedupside down and tested again, in which case its characteristics are thesame as those of conductor 50 shown in FIG. 4A.

Reference is now made to FIG. 5B, which is identical to FIG. 5A butrelates to a conductor 252, identical to conductor 52, except in that ithas a break at a location “j” therealong. FIG. 5B shows electricalpotentials induced in conductor 252 in the environment of FIG. 2,extending along first surface 42 of BUT 12, by an electromagnetic fieldgenerated by first side stimulators 14 and 16 and second side stimulator20 in the arrangement of FIG. 2. In FIG. 5B, at least one grounded metallayer 300, such as a ground plane or a power plane, extends betweenfirst and second surfaces 40 and 42 of BUT 12.

FIG. 5B includes a representation of conductor 252 arranged in spatialregistration with a first diagram 314 of the potential thereon, sensedby a sensor 25 lying above first surface 40, induced by stimulators 14and 16 as a function of the position along conductor 252 of the midpointbetween stimulators 14 and 16 along the scanning direction shown in FIG.2 and a second diagram 316 of the potential on conductor 252 induced bystimulator 20 as a function of the position along conductor 252 ofsensor 25 along the scanning direction shown in FIG. 2.

It is seen in diagram 314 that as the conductor 252 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode the potential on the conductor 252 induced bystimulators 14 and 16 is zero, inasmuch as conductor 252 is notstimulated by stimulators 14 and 16 since it extends only along secondsurface 42, which is isolated from first surface 40 by at least onegrounded metal layer 300.

It is also seen in diagram 316 that as the conductor 252 is scanned inthe scanning direction by sensor 25 lying above first surface 40, thepotential on the conductor 252 as sensed by sensor 25 remains zeroinasmuch as conductor 252 does not have any portion that extends abovegrounded metal layer 300.

It is appreciated that in order for the system of FIG. 2 to test aconductor such as conductor 252 for continuity, BUT 12 must be turnedupside down and tested again, in which case its characteristics are thesame as those of conductor 52 shown in FIG. 4B.

It is thus appreciated that the system of the embodiment of FIG. 2 doesnot provide any information regarding breaks in conductors which lieentirely along second surface 42 or entirely below grounded metal layer300, unless the BUT is turned upside down and tested again.

Reference is now made to FIG. 6A which shows electrical potentialsinduced in a typical conductor, such as conductor 58 which includesfirst portion 60 which extends along second surface 42, second portion64 located intermediate a grounded metal layer 400 and first surface 40,and third portion 68, which extends along first surface 40 of BUT 12.The electrical potentials are induced by an electromagnetic fieldgenerated by first side stimulators 14 and 16 and second side stimulator20 in the arrangement of FIG. 2 and are sensed by a sensor 25 lyingabove first surface 40.

FIG. 6A includes a representation of conductor 58, which does not haveany breaks therealong, arranged in spatial registration with a firstdiagram 420 of the potential thereon induced by stimulators 14 and 16 asa function of the position along conductor 58 of the midpoint betweenstimulators 14 and 16 along the scanning direction shown in FIG. 2 and asecond diagram 422 of the potential on conductor 58 induced bystimulator 20 as a function of the position along conductor 58 of asensor 25 along the scanning direction shown in FIG. 2.

It is seen in diagram 420 that as the conductor 58 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on conductor 58 induced by stimulators14 and 16 initially remains at a zero value inasmuch as conductor 58extends below grounded metal layer 400. When sensor 25 in thearrangement of FIG. 2 reaches second portion 64, which extends abovegrounded metal layer 400 but below first surface 40, the sensedpotential goes quickly from a zero value up to a first positive valueand then upon reaching the end of second portion 64 decreases to asecond positive value which is less than the first positive value. Uponreaching third portion 68, the sensed potential increases quickly to athird positive value, which is greater than the first positive value,and then goes to a negative value, the amplitude which is greater thanthe amplitude of the third positive value, and thereafter returnsquickly to zero. It is appreciated that when the midpoint of thestimulators 14 and 16 is over second portion 64, the maximum strength ofthe potential sensed is less than the maximum strength of the potentialsensed when the midpoint of the stimulators 14 and 16 is over thirdportion 68, thus contributing to the difference in relative amplitudes.

Turning now to diagram 422, it is seen that inasmuch as conductor 58includes first portion 60 which is located on second side 42 adjacent tosecond side stimulator 20 (FIG. 2), a potential is induced in conductor58 by stimulator 20 along the entire length of conductor 58. As seen indiagram 422, because sensor 25 only measures the potential induced onthe conductor 58 when the sensor 25 is adjacent to those portionsthereof which are above grounded metal layer 400, when sensor 25 is overfirst section 60 no potential is sensed. Potential is sensed when sensor25 is situated over second portion 64 and over third portion 68, howeverbecause second portion 64 is located at a relatively greater distancefrom sensor 25 the amplitude of the sensed potential is less than theamplitude sensed when sensor 25 is situated over third section 68.

Reference is now made to FIG. 6B, which is identical to FIG. 6A butrelates to a conductor 458, identical to conductor 58, except in that ithas a break at a location “i” therealong. FIG. 6B shows electricalpotentials induced in conductor 458 in the environment of FIGS. 2.Conductor 458 includes a first portion 460 which extends along secondsurface 42, a second portion 464 located intermediate a grounded metallayer 500 and first surface 40, and a third portion 468, which extendsalong first surface 40 and has a break therein at location “i” as shown.An electromagnetic field is generated by first side stimulators 14 and16 and second side stimulator 20 in the arrangement of FIG. 2 andelectrical potential on conductor 458 is sensed by sensor 25 (FIG. 2)which lies above first surface 40.

FIG. 6B includes a representation of conductor 458 arranged in spatialregistration with a first diagram 530 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along conductor458 of the midpoint between stimulators 14 and 16 along the scanningdirection shown in FIG. 2 and a second diagram 532 of the potential onconductor 458 induced by stimulator 20 as a function of the positionalong conductor 458 of a sensor 25 along the scanning direction shown inFIG. 2.

It is seen in diagram 530 that as the conductor 458 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on the conductor 458 induced bystimulators 14 and 16 remains at a zero value inasmuch as conductor 458extends below grounded metal layer 500. When sensor 25 in thearrangement of FIG. 2 reaches second portion 464, which extends abovegrounded metal layer 500, the sensed potential goes quickly from a zerovalue up to a first positive value and then at the end of second portion464 decreases to a second positive value, the value which is less thanthe first positive value. Upon reaching third portion 468, the sensedpotential increases quickly to a third positive value, which is greaterthan the second positive value, and then goes to a negative value andthereafter returns quickly to zero at location “i”. From location “i”,in the scanning direction, the potential on the conductor 458 induced bystimulators 14 and 16 goes quickly from zero up to a positive value andthen decreases quickly to a negative value and there after returnsquickly to zero at the end of conductor 458.

It is appreciated that there is a clear and measurable difference in thepotential pattern produced in broken conductor 458 as compared with thatproduced in continuous conductor 58.

Turning now to diagram 532, it is see that inasmuch as conductor 458includes first portion 460 which is located on second side 42 adjacentto second side stimulator 20 as shown in the arrangement of FIG. 2, apotential is induced in conductor 458 along its length until break atlocation “k”. As seen in diagram 532, because sensor 25 only senses thepotential induced on the conductor 458 when it is adjacent to theportions thereof which are above grounded metal layer 500, when sensor25 is situated over first section 460 no potential is sensed. Potentialis sensed when sensor 25 is situated over second portion 464 and overthird portion 468 until location “k”. Because of electricaldiscontinuity due to the break at location “k”, for the section of thirdportion 468 following the break in the scanning direction, which is notconnected to any portion of the conductor 458 extending below groundedmetal layer 500, no potential is induced.

Reference is now made to FIG. 6C, which is identical to FIG. 6A butrelates to a conductor 558, identical to conductor 58, except in that ithas a break at a location “l” therealong. FIG. 6C shows electricalpotentials induced in conductor 558 in the environment of FIG. 2.Conductor 558 includes a first portion 560 which extends along secondsurface 42, a second portion.564 located intermediate a grounded metallayer 600 and first surface 40, and a third portion 568, which extendsalong first surface 40. First portion 560 has a break therein as shown.An electromagnetic field is generated by first side stimulators 14 and16 and second side stimulator 20 in the arrangement of FIG. 2 andelectrical potential on conductor 558 is sensed by sensor 25 (FIG. 2)which lies above first surface 40.

FIG. 6C includes a representation of conductor 558 arranged in spatialregistration with a first diagram 630 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along conductor558 of the midpoint between stimulators 14 and 16 along the scanningdirection shown in FIG. 2 and a second diagram 632 of the potential onconductor 558 induced by stimulator 20 as a function of the positionalong conductor 558 of a sensor 25 along the scanning direction shown inFIG. 2.

It is seen in diagram 630 that as the conductor 558 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on the conductor 558 induced bystimulators 14 and 16 remains at a zero value inasmuch as conductor 558extends below grounded metal layer 600, When sensor 25 in thearrangement of FIG. 2 reaches second portion 564, which extends abovegrounded metal layer 600 but below first surface 40, the sensedpotential goes quickly from a zero value up to a first positive valueand then upon reaching the end of second portion 564 the potentialdecreases to a second positive value which is less than the firstpositive value. Upon reaching third portion 568, the sensed potentialincreases quickly to a third positive value and then goes to a negativevalue, the amplitude which is greater than the amplitude of the thirdpositive value, and thereafter returns quickly to zero. It isappreciated that because the break at location “l” is in first portion560 which lies below grounded metal layer 600, the only informationabout the presence of a break in first portion 560 is provided by theamplitude of the potential sensed at the second and third portions 564and 568.

Turning now to diagram 632, it is see that inasmuch as conductor 558includes first portion 560 which is located on second side 42 adjacentto second side stimulator as shown in the arrangement of FIG. 2, despitethe break at location “l”, some potential is induced in conductor 558along its length from break until the end of the conductor. As seen indiagram 632, because sensor 25 only measures the potential induced onthe conductor 58 when it is adjacent to those portions thereof which areabove grounded metal layer 600, when sensor 25 is situated over firstsection 560 no potential is sensed. Potential is sensed when sensor 25is situated over second portion 564 and third portion 568, howeverbecause of the break at location “i” reduces the effective size of firstportion 560 stimulated by stimulator 20, less potential is induced inconductor 568, as compared to the potential induced in correspondingunbroken conductor 58. It is appreciated that the difference may besmall and difficult to measure.

Reference is now made to FIG. 6D, which is identical to FIG. 6A butrelates to a conductor 658, identical to conductor 58, except in that ithas a break at a location “m” therealong. FIG. 6D shows electricalpotentials induced in conductor 658 in the environment of FIG. 2.Conductor 658 includes a first portion 660 which extends along secondsurface 42 below a grounded metal layer 700, a second portion 664located intermediate grounded metal layer 700 and first surface 40, anda third portion 668, which extends along first surface 40. A break isshown at location “m” in second portion 664. An electromagnetic field isgenerated by first side stimulators 14 and 16 and second side stimulator20 in the arrangement of FIG. 2 and electrical potential on conductor658 is sensed by sensor 25 (FIG. 2) which lies above first surface 40.

FIG. 6D includes a representation of conductor 658 arranged in spatialregistration with a first diagram 730 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along conductor658 of the midpoint between stimulators 14 and 16 along the scanningdirection shown in FIG. 2 and a second diagram 732 of the potential onconductor 658 induced by stimulator 20 as a function of the positionalong conductor 658 of a sensor 25 along the scanning direction shown inFIG. 2.

It is seen in diagram 730 that as the conductor 658 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on the conductor 658 induced bystimulators 14 and 16 remains at a zero value inasmuch as first portion660 of conductor 658 extends below grounded metal layer 700. When sensor25 in the arrangement of FIG. 2 reaches second portion 664, whichextends above grounded metal layer 700, the sensed potential goesquickly from a zero value up to a relatively small first positive valuedecreases to a relatively small negative value the amplitude of which isgenerally the same as the first positive value quickly increases to 0 atthe break at location m. From location “m” in the scanning direction,the sensed potential quickly increases to a second relatively smallthird positive value, and then decreases to a third positive value atthe end of second portion 664. From the beginning of the third portion668 in the scanning direction, the sensed potential quickly increases toa fourth positive value, and decreases to a negative value, theamplitude of which is greater than the fourth positive value, and thenquickly increases to zero at the end of conductor 658.

It is appreciated that there is a clear and measurable difference in thepotential pattern produced in broken conductor 658 as compared with thepotential pattern produced in continuous conductor 58.

Turning now to diagram 732, it is seen that inasmuch as conductor 658includes first portion 660 which is located on second side 42 adjacentto second side stimulator 20 as shown in the arrangement of FIG. 2, apotential is induced in conductor 658 along its length until break atlocation “m”. As seen in diagram 732, because sensor 25 only measuresthe potential induced on the conductor 658 when the sensor is adjacentto those portions which are above grounded metal layer 700, when sensor25 is situated over first section 660 no potential is sensed. Potentialis sensed when sensor 25 is situated over second portion 664 untillocation “m”. Because of the electrical discontinuity due to the breakat location “m”, for the second section of second portion 664 in theenvironment of FIG. 2 following the break at location “m” in thescanning direction and for third portion 668, neither of which have anypart extending below grounded metal layer 700, no potential is induced.

It is appreciated that there is a clear and measurable difference in thepotential pattern produced in broken conductor 658 as compared withcontinuous conductor 58.

Reference is now made to FIG. 7A, which includes a representation of aconductor 58 shown in FIG. 6A in which BUT 12 is turned upside down foradditional testing. Sensor 25 in the arrangement of FIG. 2 now lie abovesecond surface 42 of BUT 12.

FIG. 7A shows electrical potentials induced in conductor 58 which, asindicated hereinabove with reference to FIG. 6A, includes first portion60 which extends along second surface 42, second portion 64 locatedintermediate grounded metal layer 400 and first surface 40, and thirdportion 68, which extends along first surface 40. The electricalpotentials are induced by an electromagnetic field generated by firstside stimulators 14 and 16, which now lie above second surface 42, andsecond side stimulator 20, which now lies below first surface 40, andare sensed by a sensor 25 lying above first surface 42.

FIG. 7A includes a representation of conductor 58, which does not haveany breaks therealong, arranged in spatial registration with a firstdiagram 740 of the potential thereon induced by stimulators 14 and 16 asa function of the position along conductor 58 of the midpoint betweenstimulators 14 and 16 along the scanning direction shown in FIG. 2 and asecond diagram 742 of the potential on conductor 58 induced bystimulator 20 as a function of the position along conductor 58 of sensor25 along the scanning direction shown in FIG. 2.

It is seen in diagram 740 that as the conductor 58 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on conductor 58 induced by stimulators14 and 16 goes quickly from a zero value to a positive value and thendecreases to a negative value, of the same amplitude as the positivevalue, at the end of first portion 60, and quickly returns to a zerovalue. Inasmuch as second portion 64 and third portion 68 are belowgrounded metal layer 400, the potential remains at a zero value.

As conductor 58 includes third portion 64 which is located on first side40, now adjacent to second side stimulator 20 as shown in thearrangement of FIG. 2, a potential is induced in conductor 58 bystimulator 20 along the entire length of conductor 58. As seen indiagram 740, because sensor 25 only measures the potential induced onthe conductor 58 when the sensor 25 is adjacent to those portions whichare above grounded metal layer 400, a potential is sensed over firstportion 60. When sensor 25 is over second portion 64 and third portion68, no potential is sensed.

Reference is now made to FIG. 7B, which includes a representation ofbroken conductor 458 shown in FIG. 6B except that it is turned upsidedown for additional testing such that sensors 25 in the arrangement ofFIG. 2 now lie above second surface 42.

FIG. 7B shows the electrical potential induced in conductor 458 which isidentical to conductor 58 in FIG. 7A except that it includes a break atposition “k”. As indicated hereinabove with reference to FIG. 7A,conductor 458 includes first portion 460 which extends along secondsurface 42, second portion 464 located intermediate a grounded metallayer 500 and first surface 40, and third portion 468, which extendsalong first surface 40 of BUT 12. The break at a location “k” issituated in third portion 468. The electrical potential is induced by anelectromagnetic field generated by first side stimulators 14 and 16,which now lie above second surface 42, and second side stimulator 20,which now lies below first surface 40, and are sensed by sensor 25 lyingabove second surface 42.

FIG. 7B includes a representation of conductor 458, arranged in spatialregistration with a first diagram 744 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along conductor458 of the midpoint between stimulators 14 and 16 along the scanningdirection shown in FIG. 2 and a second diagram 746 of the potential onconductor 458 induced by stimulator 20 as a function of the positionalong conductor 458 of sensor 25 along the scanning direction shown inFIG. 2.

It is seen in diagram 744 that as the conductor 458 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on the conductor 458 induced bystimulators 14 and 16 goes quickly from a zero value to a positivevalue, decreases to a negative value, of the same amplitude as thepositive value, and quickly returns to a zero value at the end of firstportion 460. Inasmuch as second portion 464 and third portion 468 arebelow grounded metal layer 500, the potential of these portions sensedby sensor 25 remains at a zero value.

Turning now to diagram 746, it is seen that inasmuch as conductor 458includes second portion 464 which is located beneath metal plane 600 andthird portion 468 which is located on first side 40, now adjacent tosecond side stimulator 20 as shown in the arrangement of FIG. 2, apotential is induced in conductor 458 by stimulator 20 along the entirelength of conductor 458. However, because of the break at location “k”,the potential is somewhat reduced relative to the potential induced byunbroken third portion 68 (FIG. 7A).

As seen in diagram 746, because sensor 25 only measures the potentialinduced on the conductor 458 when the sensor is adjacent to thoseportions which are above grounded metal layer 500, a somewhat reducedpotential is sensed over first portion 460, however when sensor 25 isover second portion 464 and third portion 468, no potential is sensed.It is appreciated that there may be only small differences in thepotential induced by stimulator 20 in the configurations of FIG. 7A andFIG. 7B respectively, and that it may be difficult to differentiatebetween these differences.

It is appreciated that when tested in the “upside-down” orientation ofFIGS. 7A and 7B the difference in the potential patterns produced inbroken conductor 458 as compared with the potential patterns induced incontinuous conductor 58, namely the amplitude of the potential inducedby stimulator 20, may be difficult to measure.

Reference is now made to FIG. 7C, which includes a representation ofconductor 558 shown in FIG. 6C except that it is turned “upside-down”such that sensor 25 in the arrangement of FIG. 2 now lies above secondsurface 42 of BUT 12.

FIG. 7C shows the electrical potential induced in conductor 558 which isidentical to conductor 58 in FIG. 7A except that it includes a break atposition “1”. As indicated hereinabove with reference to FIG. 7A,conductor 558 includes a first portion 560 which extends along secondsurface 42, a second portion 564 located intermediate a grounded metallayer 600 and first surface 40, and a third portion 568, which extendsalong first surface 40. A break at a location “l” is situated in firstportion 560. The electrical potentials are induced by an electromagneticfield generated by first side stimulators 14 and 16, which now lie abovesecond surface 42, and by second side stimulator 20, which now liesbelow first surface 40, and are sensed by sensor 25 lying above firstsurface 42.

FIG. 7C includes a representation of conductor 558 arranged in spatialregistration with a first diagram 748 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along conductor558 of the midpoint between stimulators 14 and 16 along the scanningdirection shown in FIG. 2 and a second diagram 750 of the potential onconductor 558 induced by stimulator 20 as a function of the positionalong conductor 558 of a sensor 25 along the scanning direction shown inFIG. 2.

It is seen in diagram 748 that as the conductor 558 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on the conductor 558 induced bystimulators 14 and 16 goes quickly from zero up to a first positivevalue and then decreases to a negative value the amplitude of which isthe same as the first positive value, and thereafter returns quickly tozero at location “l”. From location “l”, in the scanning direction shownin FIG. 2, the potential on the conductor 558 induced by stimulators 14and 16 again goes quickly from zero up to the first positive value andthen decreases to a negative value and thereafter returns quickly tozero at the end of first portion 560. Inasmuch as second portion 564 andthird portion 568 are below grounded metal layer 600, the measuredpotential thereon remains at a zero value.

Turning now to diagram 750, it is see that inasmuch as conductor 558includes second portion 564 and a third portion 568 which are beneathgrounded metal layer 600, now adjacent to second side stimulator 20 asshown in the arrangement of FIG. 2, a potential is induced in conductor558 by stimulator 20. However, as seen in diagram 750, because of abreak at location “l”, from the beginning of first portion 560 in thescanning direction of FIG. 2 until the break at location “l”, nopotential is induced by bottom stimulator 20. Because sensor 25 onlymeasures the potential induced on the conductor 558 when adjacent tothose portions which are above grounded metal layer 600, a potential isonly sensed over first portion 560 from the break at location “l” in thescanning direction of FIG. 2 until the end of first portion 560. Whensensor 25 is over second portion 564 and third portion 568, which arebeneath grounded metal layer 600, no potential is sensed.

It is appreciated that there are a clear and measurable differences inthe potential patterns produced in broken conductor 558 as compared withthe potential patterns produced in continuous conductor 58, as sensed bysensor 25.

Reference is now made to FIG. 7D, which includes a representation ofconductor 658 shown in FIG. 6D except that it is turned “upside-down”such that sensor 25 in the arrangement of FIG. 2 now lies above secondsurface 42 of BUT 12.

FIG. 7D shows the electrical potential induced in conductor 658, whichis identical to conductor 58 in FIG. 7A except that it includes a breakat location “m”. As indicated hereinabove with reference to FIG. 7A,conductor 658 includes a first portion 660 which extends along secondsurface 42, a second portion 664 located intermediate a grounded metallayer 600 and first surface 40, and a third portion 668, which extendsalong first surface 40. A break at a location “m” is shown in secondportion 664. An electromagnetic field is generated by first sidestimulators 14 and 16 and second side stimulator 20 in the arrangementof FIG. 2 and electrical potential on conductor 658 is sensed by sensor25 (FIG. 2) which lies above first surface 40.

FIG. 7D includes a representation of conductor 658 arranged in spatialregistration with a first diagram 752 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along conductor658 of the midpoint between stimulators 14 and 16 along the scanningdirection shown in FIG. 2 and a second diagram 754 of the potential onconductor 658 induced by stimulator 20 as a function of the positionalong conductor 658 of sensor 25 along the scanning direction shown inFIG. 2.

It is seen in diagram 752 that as the conductor 658 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on the conductor 658 induced bystimulators 14 and 16 goes quickly from zero up to a first positivevalue and then decreases to a negative value, the amplitude of which isthe same as the first positive value, and thereafter returns quickly tozero at the end of first portion 660. Inasmuch as second portion 664 andthird portion 668 are below grounded metal layer 600, the measuredpotential remains at a zero value.

Turning to diagram 754, it is seen that conductor 658 includes secondportion 664 which is beneath metal plane 600 and third portion 664 whichis located on first surface 40, now adjacent to second side stimulator20 as shown in the arrangement of FIG. 2. A break is shown at location“m” in second portion 664. A relatively small potential is induced inconductor 658 by stimulator 20 along the first section of second portion664 in the scanning direction of FIG. 2, and this relatively smallpotential is sensed by sensor 25 when adjacent to first portion 660. Apotential is also induced in the second section of second portion 664 inthe scanning direction shown in FIG. 2, and in third portion 668,however when sensor 25 is over these portions, which are beneathgrounded metal layer 600, no potential is sensed thereby.

It is appreciated that when conductor 658 of FIG. 6D is tested in the“upside-down” orientation of FIG. 7D, the difference in the potentialpatterns produced in broken conductor 658 as compared with the potentialpatterns produced in continuous conductor 58 in the orientation of FIG.7A, namely the amplitude of the potential induced by stimulator 20, maybe difficult to measure.

Reference is now made to FIG. 8A which shows electrical potentialsinduced in a typical conductor, such as a conductor 770 which includes afirst portion 772 which extends along first surface 40, a second portion776 located intermediate a grounded metal layer 800 and second surface42, and a third portion 780, which extends along first surface 40 of BUT12. The electrical potentials are induced by an electromagnetic fieldgenerated by first side stimulators 14 and 16 and second side stimulator20 in the arrangement of FIG. 2 and are sensed by sensor 25 lying abovefirst surface 40.

FIG. 8A includes a representation of conductor 770, which does not haveany breaks therealong, arranged in spatial registration with a firstdiagram 830 of the potential thereon induced by stimulators 14 and 16 asa function of the position along conductor 770 of the midpoint betweenstimulators 14 and 16 along the scanning direction shown in FIG. 2 and asecond diagram 832 of the potential on conductor 770 induced bystimulator 20 as a function of the position along conductor 770 ofsensor 25 along the scanning direction shown in FIG. 2.

It is seen in diagram 830 that as the conductor 770 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on conductor 770 induced by stimulators14 and 16 quickly goes to a positive value at beginning of first portion772 and decreases to zero at the end of first portion 772. Inasmuch assecond portion 776 is beneath grounded metal layer 800, no potential issensed while sensor 25 is over second portion 776, and the potentialvalue remains zero.

Progressing in the scanning direction indicated in FIG. 2, as sensor 25reaches third portion 780, the potential decreases to a negative valueand then goes to zero at the end of third portion 780.

Turning to diagram 832, it is seen that inasmuch as conductor 770includes second portion 776 which is located beneath grounded metallayer 800 adjacent to second side stimulator 20 as shown in thearrangement of FIG. 2, a relatively small potential is induced inconductor 770 by stimulator 20 along the entire length of conductor 770.As seen in diagram 832, because sensor 25 only measures the potentialinduced on the conductor 770 when sensor 25 is adjacent to thoseportions which are above grounded metal layer 800, when sensor 25 isover first portion 772 and third portion 780, a relatively smallpotential is sensed. Inasmuch as second portion 776 is situated beneathgrounded metal layer 800, no potential is sensed when sensor 25 issituated over second portion 776 of BUT 12.

Reference is now made to FIG. 8B, which is identical to FIG. 8A butrelates to a conductor 870, identical to conductor 70, except in that ithas a break at a location “n” therealong. FIG. 85B shows electricalpotentials induced in conductor 870 in the environment of FIG. 2.Conductor 870 includes a first portion 872 which extends along firstsurface 40, and has a break as shown, a second portion 876 locatedintermediate a grounded metal layer 900 and second surface 42, and athird portion 880, which extends along first surface 40. The electricalpotentials are induced by an electromagnetic field generated by firstside stimulators 14 and 16 and second side stimulator 20 in thearrangement of FIG. 2 and are sensed by sensor 25 lying above firstsurface 40.

FIG. 8B includes a representation of conductor 870 arranged in spatialregistration with a first diagram 930 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along conductor870 of a sensor 25 situated between stimulators 14 and 16 along thescanning direction shown in FIG. 2 and a second diagram 932 of thepotential on conductor 870 induced by stimulator 20 as a function of theposition along conductor 870 of sensor 25 along the scanning directionshown in FIG. 2.

It is seen in diagram 930 that as the conductor 870 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on the conductor 870 induced bystimulators 14 and 16 quickly goes to a first positive value, decreasesto a negative value and returns to zero at the break at location “n”.From location “n” in the scanning direction indicated in FIG. 2, thepotential on conductor 870 quickly goes to a second positive value, anddecreases to a third positive value and quickly returns to zero at theend of first portion 872. Inasmuch as second portion 876 is beneathgrounded metal layer 900, no potential is sensed while sensor 25 is oversecond portion 776, and the potential value remains zero. At thebeginning of third portion 880, in the scan direction of FIG. 2, thepotential quickly goes to the third positive value, then decreases to anegative value and quickly increases to zero at the end of third portion880.

Turning to diagram 932, it is seen that inasmuch as conductor 870includes second portion 876 which is located beneath grounded metallayer 900 adjacent to second side stimulator 20 as shown in thearrangement of FIG. 2, a relatively small potential is induced inconductor 870 by stimulator 20 along conductor 870 from the break atlocation “n” until the end of third portion 880. As seen in diagram 832,because of the electrical discontinuity resulting from the break atlocation “n”, no potential is induced in first portion 872 up to breakat location “n” along the scanning direction. Sensor 25 measures thepotential induced on the conductor 870 when it is adjacent to thoseportions which are above grounded metal layer 900. When sensor 25 isover first portion 872, after location “n” in the scanning direction,and third portion 880, a relatively small potential is sensed. Inasmuchas second portion 876 is situated beneath grounded metal layer 900, nopotential is sensed when sensor 25 is situated over second portion 876of BUT 12.

Reference is now made to FIG. 8C, which is identical to FIG. 8A butrelates to a conductor 970, identical to conductor 770, except in thatit has a break at a location “o” therealong. FIG. 8C shows electricalpotentials induced in conductor 970 in the environment of FIG. 2.Conductor 970 includes a first portion 972 which extends along firstsurface 40, a second portion 976 located intermediate a grounded metallayer 1000 and second surface 42, and has a break at location “o” asshown, and a third portion 980, which extends along first surface 40.The electrical potentials are induced by an electromagnetic fieldgenerated by first side stimulators 14 and 16 and second side stimulator20 in the arrangement of FIG. 2 and are sensed by sensor 25 lying abovefirst surface 40.

FIG. 8C includes a representation of conductor 970 arranged in spatialregistration with a first diagram 1030 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along conductor970 of the midpoint between stimulators 14 and 16 along the scanningdirection shown in FIG. 2 and a second diagram 1032 of the potential onconductor 970 induced by stimulator 20 as a function of the positionalong conductor 970 of a sensor 25 along the scanning direction shown inFIG. 2.

It is seen in diagram 1030 that as the conductor 970 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, the potential on the conductor 970 induced bystimulators 14 and 16 quickly goes to a positive value at beginning offirst portion 972, decreases to a negative value and quickly returns tozero at the end of first portion 972. Inasmuch as second portion 976 isbeneath grounded metal layer 1000, no potential is sensed while sensor25 is over second portion 976, and the potential value remains zero.Progressing in the scanning direction indicated in FIG. 2, as sensor 25reaches third portion 980, the potential quickly goes to a positivevalue at beginning of third portion 980, decreases to a negative valueand quickly returns to zero at the end of third portion 980.

Turning to diagram 1032, it is see that inasmuch as conductor 970includes second portion 976 which is located beneath grounded metallayer 1000 adjacent to second side stimulator 20 as shown in thearrangement of FIG. 2, a relatively small potential is induced inconductor 970 by stimulator 20 along the conductor on either side ofbreak at location “o”. As seen in diagram 1032, because sensor 25 onlymeasures the potential induced on the conductor 970 when the sensor isadjacent to those portions of the conductor which are above groundedmetal layer 1000, when sensor 25 is over first portion 972 and thirdportion 980, a relatively small potential is sensed. Inasmuch as secondportion 976 is situated beneath grounded metal layer 1000, no potentialis sensed when sensor 25 is situated over second portion 976.

Reference is now made to FIG. 9A, which includes a representation of aconductor 770 shown in FIG. 8A in which BUT 12 is turned “upside-down”for additional testing. Sensors 25 in the arrangement of FIG. 2 now lieabove second surface 42 of BUT 12.

FIG. 9A shows electrical potentials induced in conductor 770 which, asindicated hereinabove with reference to FIG. 8A, includes first portion772 which extends along first surface 40, second portion 776 locatedintermediate grounded metal layer 800 and second surface 42, and thirdportion 780, which extends along first surface 40 of BUT 12. Theelectrical potentials are induced by an electromagnetic field generatedby first side stimulators 14 and 16 which now lie above second surface42, and second side stimulator 20, which now lies below first surface40, and are sensed by sensor 25 lying above second surface 42.

FIG. 9A includes a representation of conductor 770, which does not haveany breaks therealong, arranged in spatial registration with a firstdiagram 1040 of the potential thereon induced by stimulators 14 and 16as a function of the position along conductor 770 of sensor 25 situatedbetween stimulators 14 and 16 along the scanning direction shown in FIG.2 and a second diagram 1042 of the potential on conductor 770 induced bystimulator 20 as a function of the position along conductor 770 ofsensor 25 along the scanning direction shown in FIG. 2.

It is seen in diagram 1040 that as the conductor 770 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, inasmuch as first portion 772 is located beneathgrounded metal layer 800, the potential on conductor 770 sensed bysensor 25 when it is above first portion 772 is zero. When sensor 25reaches second portion 776 the value for the potential sensed quicklygoes to a positive value, decreases to a negative value and quicklyreturns to zero at the end of second portion 776. It is appreciated thatpotential induced in conductor 770 and sensed when sensor is over secondportion 776 is attenuated because second portion 776 is located below,and not on, second surface 42. From the beginning of third portion 780the sensed value for the potential is zero inasmuch as third portion 780is beneath the grounded metal layer 800.

Turning to diagram 1042, it is seen that inasmuch as conductor 770includes first portion 772 and third portion 780 located on first side40, now adjacent to second side stimulator 20 as shown in thearrangement of FIG. 2, a potential is induced in conductor 770 bystimulator 20 along its entire length. As seen in diagram 1042, becausesensor 25 only measures the a potential induced on conductor 770 when itis adjacent to those portions thereof which are above grounded metallayer 800, potential is sensed only over second portion 776, howeverthis potential is relatively small because second portion 776 issituated below, and not on, second surface 42. When sensor 25 is overfirst portion 772 and third portion 780, no potential is sensed becausethese portions are below grounded metal layer 800.

Reference is now made to FIG. 9B, which includes a representation of aconductor 870 shown in FIG. 8B except that it is turned “upside-down”such that sensors 25 in the arrangement of FIG. 2 now lie above secondsurface 42 of BUT 12.

FIG. 9B shows the electrical potential induced in conductor 870 which isidentical to conductor 770 in FIG. 9A except that it includes a break atposition “n” therealong. As indicated hereinabove with reference to FIG.9A, conductor 870 includes a first portion 872 which extends along firstsurface 40 and has a break therein at location “n” as shown, a secondportion 876 located intermediate grounded metal layer 800 and secondsurface 42, and a third portion 880, which extends along first surface40. The electrical potentials are induced by an electromagnetic fieldgenerated by first side stimulators 14 and 16 which now lie above secondsurface 42, and second side stimulator 20, which now lies below firstsurface 40, and are sensed by sensor 25 lying above second surface 42.

FIG. 9B includes a representation of conductor 870, arranged in spatialregistration with a first diagram 1044 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along conductor870 of the midpoint between stimulators 14 and 16 along the scanningdirection shown in FIG. 2 and a second diagram 1046 of the potential onconductor 870 induced by stimulator 20 as a function of the positionalong conductor 870 of sensor 25 along the scanning direction shown inFIG. 2.

It is seen in diagram 1044 that as the conductor 870 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, inasmuch as first portion 872 is located beneathgrounded metal layer 800, the potential on conductor 870 sensed bysensor 25 when it is above first portion 872 is zero. When sensor 25reaches second portion 876 the value for the potential sensed quicklygoes to a positive value decreases to a negative value and quicklyreturns to zero at the end of second portion 876. It is appreciated thatpotential induced in conductor 870 and sensed when sensor is over secondportion 876 is attenuated because second portion 876 is located below,and not on, said second surface 42. From the beginning of third portion880 the sensed value for the potential is zero inasmuch as third portionis beneath the grounded metal layer.

Turning now to diagram 1046, it is seen that inasmuch as conductor 870includes first portion 872 and third portion 880 located on first side40, now adjacent to second side stimulator 20 as shown in thearrangement of FIG. 2, a potential is induced in conductor 870 bystimulator 20. As seen in diagram 1046, because sensor 25 only measuresthe potential induced on conductor 870 when adjacent to those portionswhich are above grounded metal layer 800, potential is sensed only oversecond portion 876, however this potential is relatively small becausesecond portion 876 is situated below, and not on, second surface 42, andis further attenuated because the length of first portion 872contributing to the potential on conductor 870 is shortened due to breakat location “n”. When sensor 25 is over first portion 872 and thirdportion 880, no potential is sensed because these portions are belowgrounded metal layer 800.

Reference is now made to FIG. 9C, which includes a representation of aconductor 970 shown in FIG. 8C except that it is turned “upside-down”such that sensor 25 in the arrangement of FIG. 2 now lies above secondsurface 42 of BUT 12.

FIG. 9C shows the electrical potential induced in conductor 970 which isidentical to conductor 770 in FIG. 9A except that it includes a break atposition “o”. As indicated hereinabove with reference to FIG. 9A,conductor 970 includes first portion 972 which extends along firstsurface 40, second portion 976 located intermediate grounded metal layer1000 and second surface 42, and has break therein at location “o” asshown, and third portion 980, which extends along first surface 40. Theelectrical potentials are induced by an electromagnetic field generatedby first side stimulators 14 and 16 which now lie above second surface42, and second side stimulator 20, which now lies below first surface40, and are sensed by sensor 25 lying above second surface 42.

FIG. 9C includes a representation of conductor 970 arranged in spatialregistration with a first diagram 1048 of the potential thereon inducedby stimulators 14 and 16 as a function of the position along theconductor 970 of the midpoint between stimulators 14 and 16 along thescanning direction shown in FIG. 2 and a second diagram 1050 of thepotential on conductor 970 induced by stimulator 20 as a function of theposition along conductor 970 of sensor 25 along the scanning directionshown in FIG. 2.

It is seen in diagram 1048 that as the conductor 970 is scanned in thescanning direction by stimulators 14 and 16 operating in a 180 degreeout of phase mode, inasmuch as first portion 972 is located beneathgrounded metal layer 1000, the potential on conductor 970 sensed bysensor 25 when the sensor is above first portion 972 is zero. Whensensor 25 reaches second portion 976 the value for the potential sensedquickly goes to a positive value decreases to a negative value andquickly returns to zero at the break at location “o”. From break atlocation “o” in the scanning direction as indicated in FIG. 2, thepotential sensed quickly returns to a positive value, decreases to anegative value and quickly returns to zero at the end of second portion976. From the beginning of third portion 980 the sensed value for thepotential is zero inasmuch as third portion 980 is beneath the groundedmetal layer 1000.

Turning now to diagram 1050, it is seen that inasmuch as conductor 970includes first portion 972 and third portion 980 located on first side40, now adjacent to second side stimulator 20 as shown in thearrangement of FIG. 2, a potential is induced in conductor 970 bystimulator 20 along either side of break at location “o”. As seen indiagram 1050, because sensor 25 only measures the potential induced onconductor 970 when adjacent to those portions which are above groundedmetal layer 1000, the potential is sensed only over second portion 976,however this potential is relatively small because second portion 976 issituated below, and not on, second surface 42. From the beginning ofsecond portion 976 in the direction indicated in FIG. 2, the potentialsensed quickly increases to a first positive value and quickly decreasesat location “o” of the break, and thereafter again quickly increases toa positive value and quickly returns to zero at the end of secondportion 976. It is appreciated that the extent to which the potentialpatterns on either side of the break at location “o” are differentiableone from the other is a function of the size of the break. Inasmuch assensor 25 is over first portion 972 and third portion 980, no potentialis sensed because these portions are below grounded metal layer 1000.

Reference is now made to FIGS. 10A, 10B, 11A and 11B which areillustrative of the operation of the apparatus of FIG. 1 to detectshorts between conductors on a BUT 1052 in the environment shown in FIG.2.

Reference is now made to FIG. 10A which shows electrical potentialsinduced in typical conductors, such as conductors 58 and 70 in FIG. 2. Aconductor 1058 includes a first portion 1060 which extends along asecond surface 1042 of BUT 1052, a via hole 1062 connecting betweenfirst portion 1060 and a second portion 1064, which is locatedintermediate grounded metal layers 1054 and 1056, a third portion 1068which extends along a first surface 1040 of BUT 1052. A conductor 1070includes a first portion 1072 which extends along first surface 1040, asecond portion 1076 located intermediate grounded metal layers 1054 and1056, a via hole 1078 connection between second portion 1076 and a thirdportion 1080, which extends along first surface 1040. The electricalpotentials are induced by an electromagnetic field generated by firstside stimulators 14 and 16 and second side stimulator 20 in thearrangement of FIG. 2 and are sensed by sensor 25 lying above firstsurface 1040.

FIG. 10A includes a representation of conductors 1058 and 1070 which donot have any breaks therealong, and which are not mutually shorted,arranged in spatial registration with a first diagram 1130 of thepotential on conductors 1058 and 1070 induced by stimulators 14 and 16as a function of the position along conductors 1058 and 1070 of themidpoint between stimulators 14 and 16 along the scanning directionshown in FIG. 2 and a second diagram 1132 of the potential on conductors1058 and 1070 induced by stimulator 20 as a function of the positionalong the conductors of sensor 25 along the scanning direction shown inFIG. 2.

It is seen in diagram 1130 that as conductors 1058 and 1070 are scannedin the scanning direction by stimulators 14 and 16 operating in a 180degree out of phase mode, the potential on conductor 1070 induced bystimulators 14 and 16 quickly goes to a positive value at beginning offirst portion 1072 and decreases to zero at the end of first portion1072. Inasmuch as second portion 1076 lies beneath grounded metal layer1054, no potential is sensed while sensor 25 is over second portion1076, and the potential value remains zero. Progressing in the scanningdirection indicated in FIG. 2, as sensor 25 reaches third portion 1080,the potential decreases from zero to a negative value and then quicklygoes to zero at the end of third portion 1080. Further progressing inthe scanning direction of FIG. 2, inasmuch as second portion 1064 ofconductor 1058 is beneath grounded metal layer 1054, the zero value ismaintained until sensor 25 reaches third portion 1068 of conductor 1058,at which point it quickly increases to a positive value, then decreasesto a negative value and quickly increases to zero at the end ofconductor 1068.

Turning to diagram 1132, it is see that inasmuch as only first portion1060 of conductor 1058 and no portion of conductor 1070 is locatedbeneath both grounded metal layers 1054 and 1056, stimulator 20 inducesa potential only on conductor 1058, as seen in diagram 1132.

Reference is now made to FIG. 10B, which is identical to FIG. 10A exceptthat conductors 1158 and 1170 are mutually shorted at location “s”. FIG.10B shows electrical potentials induced in conductor 1158 in theenvironment of FIG. 2. As noted above, conductor 1158 includes a firstportion 1160 which extends along second surface 1042, a via hole 1162connecting between first portion 1160 and second portion 1164 which islocated intermediate grounded metal layers 1154 and 1156, and a thirdportion 1168 which extends along a first surface 1040. Conductor 1170includes a first portion 1172 which extends along first surface 1040, asecond portion 1176 located intermediate grounded metal layers 1154 and1156, a via hole 1178 connecting between second portion 1176 and thirdportion 1180, which extends along first surface 1040. Conductors 1158and 1170 are shorted at location “s” between via hole 1162 of conductor1158 and via hole 1178 of conductor 1170.

The electrical potentials are induced by an electromagnetic fieldgenerated by first side stimulators 14 and 16 and second side stimulator20 in the arrangement of FIG. 2 and are sensed by sensor 25 lying abovefirst surface 1040.

FIG. 10B includes a representation of conductors 1158 and 1170 which isarranged in spatial registration with a first diagram 1234 of thepotential on the conductors induced by stimulators 14 and 16 as afunction of the position along the conductors of the midpoint betweenstimulators 14 and 16 along the scanning direction shown in FIG. 2 and asecond diagram 1236 of the potential on conductors 1158 and 1170 inducedby stimulator 20 as a function of the position along the conductors ofsensor 25 along the scanning direction shown in FIG. 2.

Reference is now made to FIG. 11A, which includes a representation ofconductors 1058 and 1070 shown in FIG. 10A and in which BUT 12 is turned“upside-down” for additional testing. Sensor 25 in the arrangement ofFIG. 2 now lies above second surface 1042 of BUT 1052.

FIG. 11A shows electrical potentials induced in conductors 1058 and1070. As indicated hereinabove with reference to FIG. 10A, conductor1058 includes first portion 1060 which extends along second surface1042, a via hole 1062 connecting between first portion 106 and secondportion 1064, located between grounded metal planes 1054 and 1056, andthird portion 1068 which extends along first surface 1040. Conductor1070 includes first portion 1072 which extends along first surface 1040,second portion 1076 located between grounded metal planes 1054 and 1056and via hole 1078 connecting second portion 1076 and third portion 1080,which extends along first surface 1040. The electrical potentials areinduced by an electromagnetic field generated by first side stimulators14 and 16 and second side stimulator 20 in the arrangement of FIG. 2 andare sensed by a sensor 25 lying above second surface 1042.

FIG. 11A includes a representation of non-broken and non-shortedconductors 1058 and 1070, arranged in spatial registration with a firstdiagram 1238 of the potential on the conductors 1058 and 1070, as sensedby sensor 25, induced by stimulators 14 and 16 as a function of theposition along the conductors of the midpoint between stimulators 14 and16 along the scanning direction shown in FIG. 2, and a second diagram1240 of the potential on conductors 1058 and 1070 induced by stimulator20 as a function of the position along the conductors of sensor 25 alongthe scanning direction shown in FIG. 2.

It is seen in diagram 1238 that as conductors 1058 and 1070 are scannedin the scanning direction by stimulators 14 and 16 operating in a 180degree out of phase mode, the potential on conductor 1070 induced bystimulators 14 and 16, as measured by sensor 25, quickly goes to apositive value at beginning of first portion 1060, decreases to anegative value, and at the end of first portion 1060 quickly increasesto zero. It is seen that inasmuch as second portion 1064 and thirdportion 1068 of conductor 1058, are situated below grounded metal layer1054, no potential is sensed with respect to potential induced bystimulators 14 and 16 when sensor is over second portion 1064 and thirdportion 1068. It is also seen that inasmuch as conductor 1070 issituated entirely below grounded metal layer 1054, it is not stimulatedby stimulators 14 and 16 when BUT 1052 is tested in the “upside-down”orientation.

Turning now to diagram 1240, it is seen that inasmuch as conductor 1058includes third portion 1068 which is below grounded metal layer 1056,conductor 1058 is stimulated by stimulator 20. When BUT 1052 is scannedin the scanning direction, when sensor 25 is over first section 1060 thepotential induced by stimulator 20 is sensed by sensor 25. Inasmuch assecond portion 1064 and third portion 1068 of conductor 1058, arebeneath grounded metal layer 1054, sensor 25 does not sense potentialswhen over these portions. Inasmuch as all of conductor 1070 is beneathgrounded metal layer 1054, the potential induced on conductor 1070 isnot sensed by sensor 25.

Reference is now made to FIG. 11B, which is identical to FIG. 10B, butrefers to testing of BUT 1052 in “upside-down” orientation for testingas shown in FIG. 11A. FIG. 11B shows electrical potentials induced inconductors 1158 and 1170 which are mutually shorted at location “t”. Asindicated hereinabove with reference to FIG. 10B, conductor 1158includes first portion 1160 which extends along second surface 1042, avia hole 1162 connecting between first portion 1160 and second portion1164, located between grounded metal planes 1154 and 1156, and thirdportion 1168 which extends along first surface 1040 of BUT 1052.Conductor 1170 includes first portion 1172 which extends along firstsurface 1040, second portion 1176 located between grounded metal planes1154 and 1156, via hole 1178 connecting between second portion 1176 andthird portion 1180, which extends along first surface 1040. A shortexists between via hole 1162 of conductor 1158 and via hole 1178 ofconductor 1170 at location “t”. The electrical potentials are induced byan electromagnetic field generated by first side stimulators 14 and 16and second side stimulator 20 in the arrangement of FIG. 2 and aresensed by a sensor 25 lying above second surface 1042.

FIG. 11B includes a representation of conductors 1158 and 1170 arrangedin spatial registration with a first diagram 1242 of the potentialthereon induced by stimulators 14 and 16 as a function of the positionalong the conductors of the midpoint between stimulators 14 and 16 alongthe scanning direction shown in FIG. 2 and a second diagram 1244 of thepotential on conductors 1158 and 1170 induced by stimulator 20 as afunction of the position along the conductors of a sensor 25 along thescanning direction shown in FIG. 2.

It is seen in diagram 1242 that as conductors 1158 and 1170 are scannedin the scanning direction by stimulators 14 and 16 operating in a 180degree out of phase mode, the potential on conductor 1158 induced bystimulators 14 and 16, as measured by sensor 25, quickly goes to apositive value at beginning of first portion 1160, decreases to anegative value, and at the end of first portion 1160 quickly increasesto zero. It is seen that inasmuch as second portion 1164 and thirdportion 1168 of conductor 1158 are situated below grounded metal layer1156, no potential is sensed with respect to potential induced bystimulators 14 and 16 when sensor is over second portion 1164 and thirdportion 1168. It is also seen that inasmuch as conductor 1170 issituated entirely below grounded metal layer 1154, it is not stimulatedby stimulators 14 and 16 when BUT 1052 is tested in “upside-down”orientation.

Turning now to diagram 1244, it is seen that inasmuch as conductor 1158includes third portion 1168, which is below grounded metal layer 1154,conductor 1158 is stimulated by stimulator 20. As BUT 1052 is scannedalong the scanning direction, when sensor 25 is over first section 1160the potential induced by stimulator 20 is sensed. Inasmuch as secondportion 1164 and third portion 1168 of conductor 1158, are beneathgrounded metal layer 1156, sensor 25 does not sense potentials when itis over these portions. Inasmuch, as all of conductor 1170 is beneathgrounded metal layer 1154, the potential induced on conductor 1170 isnot sensed by sensor 25.

It is appreciated that when testing BUT 1054 in “upside-down”orientation, the potential patterns produced by shorted conductors 1158and 1170 is substantially the same as the potential patterns produced bynon-shorted conductors 1058 and 1070. Thus in order to identify a shortin the BUT 1054, it is necessary to perform electrical testing in“right-side-up” orientation wherein, as seen in FIG. 10B shortedconductors 1158 and 1170 are sensed to have a clear and differentpotential pattern as compared to non-shorted conductors 1058 and 1070 inFIG. 10A.

It is generally appreciated that the foregoing examples of test resultsfor various BUT configurations are not intended to encompass alltestable BUT configurations and defects, but rather they are intended toprovide illustrative examples of testing possibilities. Thus, in orderto achieve fully robust non-contact electrical testing, such aselectrical testing able to test for defects located in-between andacross internal metal layers in BUTs, BUTs are preferably non-contactelectrically tested using the aforementioned apparatus and methods byapplying stimulation and sensing, including applying stimulation to thesame side of the BUT from one or more sensors to induce potentials inthe conductors; applying stimulation to the opposite side of the BUTfrom one or more sensors also to induce potentials in conductors, andapplying stimulation and/or testing to both sides of the BUTsimultaneously and/or sequentially. If, as is preferable, stimulation isapplied simultaneously to both sides of the BUT, the stimulation isapplied so as to induce potentials which, when the electromagnetic fieldin proximity to the BUT is sensed, it is possible to distinguish betweenpotentials induced by stimulators adjacent to the first side of the BUTand potentials induced by stimulators adjacent to the second side of theBUT.

For example stimulators may be operated on both opposite sides of a BUTwhile the BUT is sensed by sensors on one or both sides thereof.Stimulation on opposite sides of a BUT may take place concurrentlyand/or sequentially. Stimulation on both sides of the BUT may be atdifferent frequencies or multiplexed. The same or different sequence ofstimulation may be used for testing a BUT in mutually “upside-down”orientations, and the BUT may be tested sequentially in substantiallyorthogonal directions.

The inventors have found that the present invention results not only ina time savings by preferably providing simultaneous performance ofnon-contact testing steps which previously could only have beenperformed sequentially, but additionally significantly increases thedetectability of faults in BUTs and reduces false alarms.

One particular advantage of the present invention lies in the fact thatby using sensors each of which simultaneously detects potentials inducedby multiple, separable electromagnetic stimuli, the potential patternsgenerated are automatically spatially registered thus generallyobviating the need for further spatial registration. As a result,distribution patterns of potentials received in respect of stimulationprovided on the same side of the BUT as on which the potentials aresensed are easily correlated with distribution patterns of potentialsreceived in respect of stimulation provided on the opposite side of theBUT from that on which the potentials are sensed.

Reference is now made to FIGS. 12 and 13, which are schematicillustrations of two alternative preferred configurations of first sidestimulators 14 and 16 and second side stimulator 20. The configurationsillustrated in FIGS. 12 and 13 are designed to reduce interferencedifficulties, such as capacitance interference as may be caused byconductors which cross through metal layers such as first metal layer 54and second metal layer 56 (FIG. 2).

Referring to FIG. 12, it is seen that a stimulator 1310 is preferablysectioned into a plurality of mutually aligned linear stimulation strips1312, which are oriented perpendicular orientation to the scanningdirection along which a BUT, such as BUT 12 shown in FIG. 2, is scanned.

Turning now to FIG. 13, a stimulator 1320 may be partitioned into amultiplicity of individual stimulator patches 1322, each patch being aseparate individually controllable antenna. As is readily appreciated,strips 1312 of stimulator 1310, as shown in FIG. 12, and stimulationpatches 1322 of stimulator 1320 as shown in FIG. 13 are preferablyprovided with AC stimulation signals of different frequencies ormultiplexed inputs, in order to enable separation of potentials inducedby individual strips 1312 or patches 1322 respectively.

Reference is now made to FIG. 14 which is an alternative configurationof non-contact electrical testing apparatus constructed and operative inaccordance with a preferred embodiment of the present invention. Theapparatus of FIG. 14 may be similar to that shown in FIG. 1 and differstherefrom in the arrangement of the stimulators and the sensors.

As seen in FIG. 14, there is provided testing apparatus 1410, which isoperative to perform non-contact electrical testing of electricalcircuits, such as are found on BUT 12, having a multiplicity ofelectrical conductors 13. Testing apparatus 1410 comprises an array ofstimulator electrodes 1414, including a multiplicity of individuallycontrolled stimulators 1416 linearly disposed adjacent to a first sideof BUT 12. A signal generator 1417 supplies an electrical stimulationsignal to each of the stimulators 1416. Preferably the stimulationsignals are at different frequencies, or multiplexed.

First side sensor electrodes, hereinafter referred to as sensors 1418and 1420 are arranged on opposite sides of array 1414 to lie adjacent afirst side of BUT 12. A second side sensor electrode, hereinafterreferred to as sensor 1422, is arranged to underlie BUT 12 on a sidethereof opposite first side sensors 1418 and 1420. Each of the sensors141 , 1420 and 1422 is preferably at least as large as a BUT.

A separating detector 1426 receives the outputs of each sensor 1418,1420 and 1422 respectively, and supplies the outputs to a signalanalyzer 1428 which outputs to a comparator and report generator 1430.

As noted hereinabove, the AC signals provided by signal generator 1417to each stimulator 1416 in stimulator array 1414 are preferablydifferent. This may be accomplished either by providing signals ofdifferent frequencies or alternatively by multiplexing or other knownsignal differentiation methods.

When energized by the AC electrical stimulation signals, stimulators1416 generate localized electromagnetic fields which stimulate variousconductors on BUT 12. It is appreciated that each stimulator induces acharacteristically different measurable potential. Sensors 1418, 1420and 1422 sense the different potentials and preferably output to theseparating detector 1426, which is operative to separate out each of thepotentials.

In a preferred embodiment of the present invention, BUT 12 and secondside sensor 1422 are moved linearly past stimulator array 1414 andsensors 1418 and 1420. It is readily appreciated that alternatively BUT12 and sensor 1422 may be held stationary while stimulator array 1414and sensors 1418 and 1420 are moved. Other combinations may also besuitable to scan BUT 12 with stimulator array 1414.

As in the apparatus described with respect to FIG. 1, by employinginformation indicating potentials at various locations on BUT 12 sensedby sensors 1418, 1420 and 1422, signal analyzer 1428 generates a preciserepresentation characteristic of potentials in conductors 13 on BUT 12,which indicates, inter alia, conductor continuity and which includesinformation regarding shorts and breaks in conductors, which constitutedefects.

The representation provided by signal analyzer 1428 to comparator andreport generator 1430 enables provision of a defect report 1431indicating defective electrical continuity in conductors 13 of BUT 12.

Turning now to FIG. 15, there is shown a schematic circuit diagram of apreferred embodiment of a separating circuitry 1440 useful in separatingdetector 1426 (FIG. 14). Outputs from each of top side sensors 1418 and1420 are supplied to first and second amplifiers 1442 and 1444respectively, which are input into a differential amplifier 1446.

The output of differential amplifier 1446 is provided to a mixer 1448which also receives a signal input from signal generator 1417 andoutputs to a low-pass filter (LPF) 1450, operative to remove undesirableAC out-band signal portions. The output of LPF 1450 is a DC voltagerepresentative of the potential sensed on BUT 12 by a sensor 1418 or1420 at a predetermined frequency, which is proportional to the relativeamplitude of the signal input components input from sensor 1418 and 1420respectively, each frequency being correlated to a given one ofstimulators 1416, thereby to provide spatial information.

An output from bottom side sensor 1422 is supplied to an amplifier 1452which outputs to a mixer 1454, which also receives a signal input fromsignal generator 1417. The output of mixer 1454 is provided to alow-pass filter (LPF) 1456, operative to remove undesirable AC out-bandsignal portions. The output of LPF 1456 is a DC voltage representativeof the amplitude of the signal input of a potential at a predeterminedfrequency sensed on BUT 12 by sensor 1422.

It is readily appreciated that separate circuits may be provided foreach of the frequencies at which stimulators 1416 stimulate BUT 12, or alesser number of circuits may be employed, in which event multiplexingof signals to sensors 1416 is required.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been specifically describedhereinabove. Rather the scope of the present invention includes bothcombinations and subcombinations of various features describedhereinabove as well as modifications and variations thereof which wouldoccur to a person skilled in the art upon reading the foregoingdescription and which are not in the prior art.

1-82. (canceled)
 83. Apparatus for electrically testing electricalcircuits comprising: at least one array of non-contact stimulatorelectrodes including a multiplicity of individually controlledstimulator electrodes arranged to be linearly disposed adjacent a firstside of an electrical circuit to be tested; a signal generator coupledto said at least one array arranged to supply an electrical stimulationsignal to each of the stimulator electrodes; and at least twonon-contact sensor electrodes, each sensor electrode having dimensionssufficiently large to overlay part of a conductor on said electricalcircuit to be tested.
 84. Apparatus as claimed in claim 83, wherein saidat least one of said two non-contact sensor electrodes is arranged tolie on a second side of said electrical circuit to be tested, oppositeto said first side.
 85. Apparatus as claimed in claim 83, wherein saidsensor electrodes are operative to correlate a signal to a particularnon-contact stimulator electrode to provide spatial information. 86.Apparatus as claimed in claim 83, wherein at least some of saidelectrical stimulation signals are at different frequencies. 87.Apparatus as claimed in claim 83 wherein said electrical stimulationsignals are multiplexed.
 88. Apparatus as claimed in claim 83, whereinsaid at least two non-contact sensor electrodes are arranged to lieadjacent said at least one array of non-contact stimulator electrodes.89. Apparatus as claimed in claim 88, wherein said at least twonon-contact sensor electrodes are arranged to lie on opposite side ofsaid at least one array of non-contact stimulator electrodes. 90.Apparatus as claimed in claim 83, wherein said at least two non-contactsensor electrodes includes at least one sensor electrode arranged to lieadjacent a second side of said electrical circuit to be tested, saidsecond side being opposite said first side.
 91. Apparatus as claimed inclaim 83, further comprising: a separating detector arranged to receivean output from each of said non-contact sensor electrodes and beingoperative to correlate a signal to a particular non-contact sensorelectrode; a signal analyzer operative to receive said outputs and toanalyze the outputs; a comparator operative to compare said outputs toan expected signal; and a report generator at least reporting thepresence of defects in said electrical circuit to be tested. 92.Apparatus as claimed in claim 91, wherein said defects included defectsselected from a group of defects including: faulty conductor continuity,shorts between conductors, and breaks in conductors.
 93. Apparatus asclaimed in claim 83, wherein said non-contact stimulator electrodes areconfigured to generate localized electromagnetic fields each stimulatingdifferent conductors on said electrical circuit to be tested. 94.Apparatus as claimed in claim 83, wherein said non-contact stimulatorelectrodes are arranged to be scanned over said electrical circuit to betested.
 95. Apparatus as claimed in claim 83, wherein said non-contactsensor electrodes are at least as large as said electrical circuit to betested.
 96. A method for electrically testing electrical circuits,comprising: stimulating conductors on an electrical circuit to be testedwith a multiplicity of individually controlled stimulator electrodeslinearly arranged adjacent a first side of said electrical circuit to betested; supplying an electrical stimulation signal to each of thestimulator electrodes; and sensing a response to said stimulating withat least two non-contact sensor electrodes, each sensor havingdimensions sufficiently large to overlay part of a conductor on saidelectrical circuit to be tested.
 97. The method as claimed in claim 96,further comprising correlating a signal to a particular non-contactstimulator electrode to provide spatial information.
 98. The method asclaimed in claim 97, wherein said correlating comprises operating saidstimulator electrodes at different frequencies.
 99. The method asclaimed in claim 97, wherein said correlating comprises multiplexingsaid electrical stimulation signals.
 100. The method as claimed in claim96, wherein sensing comprises sensing said response on said first sideof said electrical circuit to be tested.
 101. The method as claimed inclaim 100, wherein said sensing comprises sensing said response onopposite sides of said multiplicity of said non-contact stimulatorelectrodes.
 102. The method as claimed in claim 96, wherein sensingcomprises sensing said response on a second side of said electricalcircuit to be tested, said second side being opposite said first side.103. The method as claimed in claim 96, further comprising: associatinga signal with a particular non-contact sensor electrode; analyzingoutputs of said sensors; comparing compare said outputs to an expectedsignal; and reporting the presence of electrical defects in saidelectrical circuit to be tested.
 104. The method as claimed in claim103, wherein said defects included defects selected from a group ofdefects including: faulty conductor continuity, shorts betweenconductors, and breaks in conductors.
 105. The method as claimed inclaim 96, wherein stimulating comprises generating localizedelectromagnetic field stimulating a different conductor on saidelectrical circuit to be tested.
 106. The method as claimed in claim 96,further comprising scanning said non-contact stimulator electrodes oversaid electrical circuit to be tested.
 107. The method claimed in claim96, wherein said non-contact sensor electrodes are at least as large assaid electrical circuit to be tested.